Mysterious Ribbon at Edge of Solar System is Changing

A year ago, researchers from the IBEX mission – NASA’s Interstellar Boundary Explorer – announced the discovery of an unexpected bright band or ribbon of surprisingly high energy emissions at the boundary between our solar system and interstellar space. Now, after a year of observations, scientists have seen vast changes, including an unusual knot in the ribbon which appears to have ‘untied.’ Changes in the ribbon — a ‘disturbance in the force,’ so to speak, along with a shrunken heliosphere, may be allowing galactic cosmic rays to leak into our solar system.

“We didn’t understand where the ribbon came from in the first place,” said David McComas, IBEX principal investigator, during a press briefing. “It’s even more confounding now, to know the structure can change on incredibly short timescales.”

Researchers believe the ribbon forms from the interactions between interstellar space and the heliosphere, the protective bubble in which the Earth and other planets reside. The heliosphere is inflated by the solar wind, and acts as a protective shield from galactic cosmic rays that would otherwise bombard planets and perhaps prohibit life.

The interaction of the solar wind and interstellar medium creates energetic neutral atoms of hydrogen, called ENAs, that zip away from the heliosheath in all directions. Some of these atoms pass near Earth, where IBEX records their arrival direction and energy. As the spacecraft slowly spins, the detectors gradually build up pictures of the ENAs as they arrive from all over the sky.

IBEX produces global maps of the outer region of our solar system every six months. From the first map of the ribbon, released a year ago, scientists saw the unexpected ribbon, with a knot feature seen in the northern portion of the ribbon, as the brightest feature at higher energies.

The new, just-released map shows the large-scale structure of the ribbon, and another surprise: the distribution changed significantly. Overall, the intensity of ENAs has dropped 10% to 15%, and the hotspot has diminished and spread out along the ribbon.

McComas says the dropoff in intensity between the two all-sky maps perhaps makes sense, because the Sun is only now emerging from an unusually long period of very low activity and a correspondingly weak solar wind. The fewer the solar-wind particles that reached the heliosphere in recent years, meant the bubble may have shrunk. A smaller bubble allows more galactic cosmic rays can make their way into the inner solar system

“If we’ve learned anything from IBEX so far, it’s that the models we were using for interaction of the solar wind with the galaxy were just dead wrong,” said McComas.

With previous missions in our solar system, scientists have been able to get a handle on the medium inside termination shock, learning about the solar wind and how it is tied to structures on the sun.

“IBEX is first mission to give us definitive information about the medium that lies just beyond the heliosphere,” said Nathan Schwadron IBEX science operations lead. “The ribbon is tied in some ways to direct the orientation of the local field, giving us constraints how galactic medium affects the whole system. This is critical information that we have been missing.”

The scientists are hopeful IBEX will continue to operate through an entire solar cycle so that they can track the changes in the ribbon as solar activity is expected to increase in the next few years.

The paper, “The evolving heliosphere: Large-scale stability and time variations observed by the Interstellar Boundary Explorer,” was published online Sept. 29 in the American Geophysical Union’s Journal of Geophysical Research.

The lead video shows how the heliosphere is a bubble that surrounds our entire solar system and is inflated by the outward solar wind, which pushes out and deflects the material from the part of the galactic medium through which our Sun and solar system continually moves. This animation starts at our sun and quickly zooms out from the solar system to reveal the heliosphere and its collision with interstellar gas. The two Voyager spacecraft are currently exploring this interaction region. Credit: Goddard Conceptual Image Lab/Walt Feimer

Sources: NASA, Southwest Research Institute